While this invention was developed for use in communicating data between various avionic systems and subsystems that need to share data, and is described in such an environment, it is to be understood that the invention can be utilized to communicate binary data in other environments. It is also to be understood that while the invention was developed for use with a current mode data bus, and is described in connection with such a bus, many of the aspects of the invention can be utilized in connection with other types of electric wire and other data buses to improve the operation thereof; in particular, voltage mode and optical data buses. Similarly, while the invention was developed for use in a data communication system wherein the binary data to be communicated is coded in Manchester biphase form, it is to be understood that the invention can be used with binary data coded in other rectangular forms, such as binary data coded in mark-space form.
In modern aircraft, it is desirable to integrate, as far as possible, the functions of previous wiring-independent avionic systems to permit an attendant reduction in the weight, space and power requirements of the avionic systems, and to permit a simplification in wiring between physically separated avionic systems or subsystems thereof. Such integration has been achieved by the use of a common data bus to which each avionic system, or a subsystem thereof, has access through an associated terminal, each of which is capable of transmitting and receiving data. Data transmitted on the data bus by one terminal associated with a particular system or subsystem can be received by the terminals associated with remaining systems or subsystems, thus eliminating the requirement for separate wiring interconnections between the systems or subsystems. In addition, data generated by a particular system or subsystem can be used by any other system or subsystem without the necessity of having to independently generate that data.
While various types of data communication systems have been developed for use onboard aircraft to communicate between avionic systems and subsystems, as Z described in U.S. Pat. Nos. 4,199,663 and 4,471,481, both entitled "Autonomous Terminal Data Communications System" and assigned to the assignee of the present application, the most desirable avionic data communication system is an autonomous terminal data communication system; in particular, an autonomous data terminal communication system that uses a current mode data bus. Items critical to the operation of a data communication system that utilizes a current mode data bus are the reliability of the bus cable and the efficiency and reliability associated with the way each terminal is coupled to the bus. Current mode data bus coupling efficiency and reliability is addressed in U.S. Pat. No. 4,264,827 entitled "Current Mode Data or Power Bus," also assigned to the assignee of the present application. The essence of the invention described in this patent is a coupling transformer having a ferrite core designed such that the core can be disassembled and the two wires of a bus formed by a pair of twisted wires placed around the legs Of the core in such a way that the magnetic path of the reassembled core surrounds the conductors. The arrangement is such that the bus wires form one of the windings of a transformer. The other winding is permanently installed On the core and is connected to the data transmitter and/or receiver electronics of a data terminal. The end result is the establishment of current coupling without the need to cut the bus wires or remove or perforate the insulation that surrounds the wires.
While a coupling transformer of the type described in U.S. Pat. No. 4,264,827 is highly reliable, in order to optimize the benefits of a data communication system using a current mode data bus and such transformers, it is necessary that the coupler transformer circuitry, i.e., the circuit that applies data signals to the transformer for application to the current mode data bus and the circuit that receives data signals from the transformer, meet certain criteria. These criteria can be best understood by considering certain similarities and differences between a voltage mode data bus and a current mode data bus formed of a pair of twisted wires. As illustrated in FIG. 1, signal propagation from point A towards the end of a bus formed of a pair of twisted wires is the same for a voltage mode data bus and a current mode data bus. That is, the signal propagates along the data bus from the point where it is applied, toward the end(s) of the bus. For best results, the output impedance of the data transmitter should equal the characteristic impedance of the bus. Further, the bus wires should be terminated by a resistor, R.sub.0, whose impedance equals the characteristic impedance of the bus so that signal reflections are avoided.
As shown in FIG. 2, transmitted data signals, V.sub.c, are applied to a voltage mode data bus across the bus wires. V.sub.c drives current I.sub.1 through terminating resistor R.sub.1 in one direction, and current I.sub.2 through terminating resistor R.sub.2 in the opposite direction. Thus, the total current flow created by V.sub.c, i.e., I.sub.c, is equal to I.sub.1 plus I.sub.2. As shown best in an equivalent circuit (FIG. 3), R.sub.1 and R.sub.2 are connected in parallel. If R.sub.1 and R.sub.2 are the same, they can both be set equal to R.sub.0 whereby: I.sub.c =2V.sub.c /R.sub.o and I.sub.1 =V.sub.c /R.sub.o.
FIG. 4 illustrates a current mode data bus wherein half of the coupler voltage, V.sub.c, is applied between points C and D located on one of the bus wires of a twisted wire pair that forms the data bus and the other half is applied between points E and F located on the other bus wire. The equivalent circuit is shown in FIG. 5. In this circuit I.sub.1 =I.sub.2 =I.sub.c. Further, since R.sub.1 and R.sub.2 are in series, the following equations apply: I.sub.c =V.sub.c /2R.sub.o and I.sub.1 =V.sub.c /2R.sub.o.
The foregoing discussion leads to certain conclusions about a current mode data bus. First, the voltage of signals applied to a current mode data bus must be twice the voltage of signals applied to a voltage mode data bus to create the same current level in both buses. Second, the output impedance of a current mode data bus signal source must be low when a signal is not being applied in order to avoid loading the data bus. Third, the input impedance of the signal receivers coupled to a current mode data bus must be low for the same reason, i.e., to avoid loading the data bus. The second and third conclusions follow from the fact that, rather than applying an impedance in parallel across the bus wires as in a voltage mode data bus coupler (FIGS. 2 and 3), a current mode data bus coupler applies a series impedance to the bus wires. Bus loading by a parallel impedance is avoided by making the impedance high. Bus loading by a series impedance is avoided by making the impedance low.
As will be better understood from the following description, the present invention provides a current mode data bus based data communication system that functions in accordance with the three criteria discussed above. More specifically, the invention provides transmit couplers that apply relatively high voltage data signals to the current mode data bus and have a low output impedance during inactive periods. The invention also provides receive couplers that have a low input impedance.
The preferred form of a current mode data bus based data communication system formed in accordance with the invention accomplishes the foregoing results in a way that minimizes weight and size by minimizing the volume and area of the core of the transformers of the transmit and receive couplers. Core volume is significant because core losses are a function of core volume. Up to the limiting factor of core saturation, minimizing core volume minimizes core losses. Core area with respect to core saturation is a function of signal frequency. Because a signal with some low frequency components will saturate a transformer core of fixed size before a signal with only higher frequency components, maintaining signal frequency high minimizes core area (and volume). A high signal frequency also minimizes signal drop in signal receiver circuits.